Differential Protection of 3 phase Induction Motors
- Ninad Deo
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS
This article covers the basics of differential protection and some basics of the differential protection of motors as a special case. The general topics include:
The basics and necessity of differential protection in motors Types of differential protection - Simple differential protection - Percent Differential Protection Differential relays - Settings - typical specifications Differential C.T’s - Typical specifications
The following is out of scope of this article and will be covered in later articles:
Differential C.T’s ( selection/type/application/working and testing) Differential C.T’s (Design) Differential relays ( selection/type/application/working/detailed settings) Differential protection using V.T’s.
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS
Differential Protection is based on the fact that any fault within an electrical equipment would cause the current leaving it to be different that the current entering it. Thus by comparing the two currents either in phase or magnitude or both, a trip output can be issued if the difference exceeds a predetermined set value. Differential protection is especially attractive when both ends of the apparatus are physically located near each other. For example, this method is commonly used in Transformer/Generator/Busbar/motor protection. We shall deliberate on the differential protection used specifically in motor protection schemes as our product pertains to the same. Types of faults occurring within the protected zone requiring immediate tripping and isolation of the motor are: a. faults between stator windings b. stator earth faults c. Ground faults and faults between phases outside the generator but within the protected zone, e.g. at the generator terminals or on the external connections. An extremely important feature of any motor differential protection is that it should remain absolutely stable (i.e. no tripping command) for faults or any other transient phenomena outside the protected zone. If for a motor, the motor kVA rating is less than half of the supply transformer kVA rating, over current relays may be relied upon. However, in case of high voltage motors (commonly called as “big” motors), whose kVA rating is more than half of the supply transformer kVA rating, the current for a 3 phase fault may be less than 5 times the current for locked rotor condition. In such cases, it is recommended to use percentage differential protection (explained further).The reason can be given with a simple example: Assume a motor is connected to a supply transformer with 8 % impedance. The maximum fault current at the transformer secondary with an infinite source is = 1 / 0.08 = 12.5 p.u. on transformer base The maximum motor starting current in this case is = 1 / (0.08 + Xm) Where Xm is motor impedance In order that / > 5, Xm> 0.32 p.u. (on transformer kVA base). If the motor has full voltage starting current of six times the full load, then Xm = 1/6 = 0.167 on motor rated kVA base. With the motor kVA half of the transformer kVA, an Xm of 0.167 would be 0.333 on transformer base which is greater than 0.32. It is to be noted that this rule of thumb should only be applied when there is no appreciable deviation from the parameters assumed above.
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS
The simple differential protection scheme : This scheme is also known as the “Merz-Price scheme”. The currents entering and leaving the equipment to be protected are stepped down with the help of C.T’s on either side. The following rule can be applied for the dot notation: when the current enters the dotted terminal on the primary side of the C.T, it should leave the similarly marked dotted terminal on the secondary side. For normal condition, the currents transformed by the C.T being equal in phase and magnitude just circulate on the secondary side and there is spill current in the over current relay. The over current relay is wired to trip the two circuit breakers on either side of the equipment to be protected. A differential relay (over current relay) responds to the vector difference between two or more similar electrical quantities. Thus the simple differential relay is stable during normal operating condition. Similarly during external fault condition, there is no difference in phase or magnitude of the entering and leaving current thus giving a zero spill current. Thus the simple differential relay is stable for the external fault condition also.
Protected winding I1
I2
x
I1
I2
I1 – I2 =0
I1 = I2 For normal condition And through fault
Relay coil
Principle of circulating current relay for protection of motors
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS
Our Interest: Behavior during internal fault For internal fault, due the difference in the entering and leaving values of the current in and out of the C.T, there is a spill current in the over current relay which if more than the pick-up value of the over current relay will cause the circuit breakers to trip thus meeting the basic requirement of clearing internal faults.
Protected winding I1
I2
x
I1
I2
I1 – I2 =0
I1 = I2 For normal condition And through fault
Relay coil
Principle of circulating current relay for protection of motors
Difficulties and errors in the Simple Differential Protection scheme : 1. Pilot wire lengths : The C.T’s and machine to be protected are located at different sites and normally it is not possible to connect relay coil to the equipotential points. The difficulty is overcome by connecting adjustable resistors in series with the pilot wires. These are adjusted on site to obtain equipotential points. 2. CT errors during short circuit : The CT’s may have almost equal ratio as normal currents. But during short circuit conditions, the primary currents are unduly large. The ratio errors of CT’s on either sides differ during these conditions due to: a. Inherent difference in CT characteristic arising out of difference in magnetic circuit, saturation conditions etc. b. Unequal d.c components in the short circuit currents 3. Saturation of CT magnetic circuits during short circuit conditions : Due to these causes the relay may operate even for external faults. The relay may lose its stability for through faults. To overcome this difficulty the “percentage differential relay” or “Biased differential relay” is used. It is essentially a circulating current differential relay with additional restraining coil. The current flowing through the restraining coils is proportional to (I1+I2)/2 and this restraining current prevents the operation during
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS external faults. Because with the rise in current the restraining torque increases and I1-I2 arising out of difference in CT ratio is not enough to cause relay operation.
External Fault characteristic
2 I – 1 I
Pick up value of OC relay in spill path
t n e r r u c l l i p S
Trip
Ips Restrain
Through fault current
Istab Through Fault stability
Operating characteristic of a simple differential relay
The through fault stability is defined as the maximum through fault current beyond which the scheme loses stability. It is denoted as: Comparing this with an internal fault, the minimum internal fault current required for the scheme to operate, in this case, correctly, is decided by the pick-up value of the over current relay in the spill path. Then to have a better idea of the relation of this value and we define a term called as the stability ratio which is given by:
The higher the stability ratio, the better is the system able to distinguish between internal and external faults. The stability ratio can be improved by improving the match between the two Current transformers.
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS
The percent differential protection scheme : The simple differential relay can be made more stable if one can develop a restraining torque proportional to the fault current, the operating torque still proportional to the spill current. This is implemented in the percent differential relay. This relay has a coil tapped at the centre thus forming 2 sections with equal number of turns: Nr/2. The restraining coil receives the “though fault” current since it is connected in the circulating current path. The operating coil having No turns is connected on the spill path same as the simple differential relay. Then obviously,
-
Where M is constant of proportionality.
Similarly,
And the relay trips if the operating torque is greater than the restraining torque. Equating the restraining and operating torques, with K = following equation:
/ , we come to the
Where accounts for the effect of spring. Thus the operating characteristics of this relay will be a straight line with a slope of / and an intercept of on y axis. The characteristic is shown in the figure.
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS
Protected zone I1
I2
fault
I1 = I2 = 0 For internal fault
I1
I2
I1 – I2 I2
I1 N/2
N/2 Restraining coil/ Biased coil
Percent Differential Relay
Spring
Trip Output
C
Armature
Operating coil N0 Restraining coil
A Nr/2 Nr/2 B
Balanced beam Structure
Physical Implementation of a Percentage differential Relay
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS
Internal Fault Characteristic 200 % slope
2 I – 1 I t n e r r u c l l i p S
Minimum pick up
Internal Faults
External Fault characteristic
% differential relay Characteristic 25% slope
Trip Restrain Simple differential relay characteristic
(I1 + I2) /2 Average Restraining current Minimum internal Fault current (If min, int) Maximum through fault current (If,max,ext)
Operating characteristic of a percentage differential relay
Thus the spill current must be greater than a definite percentage of the “through fault” current for the relay to operate. Hence the name percent differential relay. The slope of the relay is expressed as percentage. E.g. slope of 0.4 is 40 % slope. One of the most important things to note is that the percentage differential relay does not have a fixed pick-up value as it automatically adapts the pick-up value to the “though fault” current. Thus as the through fault current goes on increasing, a restraining torque is introduced due to the circulating current thus avoiding “mal-operation” of the relay. The restraining winding is also known as the biasing winding since we bias the relay towards restraint. The slope of the characteristic is also known as percentage bias. The percentage differential relay can be made more immune to mal-operation on “though fault” by increasing the slope of the relay.
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS
Specifically for a 3 phase Induction Motor, the following connections are done as shown in figure: Induction motor
e s y a l h p p p u 3 s
3 units of Percentage Differential Relay
Percentage Differential Protection for3 phase IM
For instance, Innovative Technomics has provided Dynamic compensator differential C.T’s for IOCL Haldia project in conjunction with motor protections C.T’s. The typical connection diagram for secondary side connection is shown:
A typical differential C.T secondary side connection for Induction motor protection in motor and soft starter combination with soft starter with dynamic compensator provided by ITPL
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS
Differential Relays:
The basis of differential protection lies in monitoring the spill current which is done by differential relays. The operation of the relays is based on the settings on which the relays are operated. Setting of Differential Relays The circulating current differential relay has two principle settings namely: - Setting of operating coil circuit - Setting of restraining coil circuit The percentage setting or basic setting of the operating coil circuit is defined as the ratio:
X 100
- When the current in restraining coil is zero The setting of operating coil or its pick-up value is defined as the ratio:
When determining this setting the following factors must be considered: - CT errors - Tap changing - Resistance of pilot wires - Stability for through faults
DIFFERENTIAL P OTECTION OF 3 PHASE INDUCTION MOTO RS
For a typical percentage differential relay a block diagram showing the s ettings is shown:
The slope setting is adju ted by the tapping on the restraining coil. Very important: Both ha lves of the restraining coil need to be symmetri ally tapped. Minimum pick-up setting is adjusted by changing the tension of the restr aining spring
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS
A typical specification of a differential relay A typical specification of a microprocessor based differential relay also known as a “Residual current detector” or RCD is given below:
Static, three-phase differential protection relay - Dual slope percentage bias restraint characteristic with adjustable bias setting - Electronically storage for indication of the faulty phase Applicable for 45 to 65 Hz - Burden < 0.05 VA at rated current
-
Setting ranges: Differential current: 5 to 42.5 % in 16 steps Bias slope: 10 % of actual current (fixed)
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Isolation between all independent inputs High electromagnetic compatibility The use of precision components guarantees high accuracy Permissible temperature range: -20°C to +70°C
Some manufacturers also give special and extra functions inbuilt in the relays like: - Ability to recognize saturation of the main current transformers Extremely stability even during saturation of current transformers Current transformer burden and class requirements are low Extremely stability during motor start - Additional printed circuits for recognition of saturated C.T.s can be added at a later stage, e.g. as the power system develops and fault levels increase
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS
A typical Static differential protection scheme functional diagram
DIFFERENTIAL PROTECTION OF 3 PHASE INDUCTION MOTORS
A typical Specification for a Differential C.T A differential protection C.T is different than the “measurement’ C.T in the basic sense that the protection C.T is required to faithfully transform the primary current even in presence of a fault whereas the measurement C.T is driven into saturation in the presence of a fault. This is done by the C.T manufacturers by fixing the operating point of the C.T on the magnetization curve suitable to its operation. A typical specification of differential C.T’s used by Innovative Technomics in IOCL Haldia project: Differential C.T (Indoor Resin Cast single core current transformer) C.T Ratio: 500/1 A RCT: 8 ohm = 110V < 30mA at /2 Rated STC: 40 kA for 3 seconds 50 Hz Insulation level: 12/28/75 kV Accuracy class: PS to IS 2705/1992
The C.T selection/application/testing and basic design is out of scope of this article and will be covered in later articles.
